High-speed precision positioning apparatus

ABSTRACT

A high-speed precision positioning apparatus has a stage supported by a platen. The stage is driven by a plurality of drive motors that are co-planar with the stage and arranged symmetrically around the stage. The drive motors apply drive forces directly to the stage without any mechanical contact to the stage. The drive forces impart planar motion to the stage in at least three degrees of freedom of motion. In the remaining three degrees of freedom the motion is constrained by a plurality of fluid bearings that operate between the stage and the platen. The drive motors are configured as magnets, attached to the stage, moving in a gap formed in-between top and bottom stationary coils. Integral force cancellation is implemented by a force cancellation system that applies cancellation forces to the stage. The cancellation forces, which are co-planar with a center of gravity of the stage and any components that move with the stage, cancel forces generated by planar motion of the stage. Interferometric encoders are used as position detectors.

This is a continuation of U.S. patent application Ser. No. 09/156,895,filed Sep. 18, 1998.

BACKGROUND OF THE INVENTION

This invention relates to high-speed precision positioning apparatusesuseful for processing of devices such as semiconducting wafers, formicro-scale experimentation, or for high resolution electron microscopyof biological or other samples.

Semiconductor processing equipment often requires high-precisionpositioning of a silicon wafer. Large-size wafers with very smallfeatures often require high-precision positioning combined with a largerange motion of the wafer stage that supports the wafer.

In addition to high precision and a large range motion, high-speedscanning is useful for achieving high manufacturing throughput.

A typical example of semiconductor processing equipment is a laser usedin microlithography. Certain traditional systems for semiconductor laserprocessing include a translation stage that can support the wafer andmove it under a fixed laser beam. The wafer-supporting translation stagemay move along the X or Y directions, for example.

In certain processing equipment a linear motor and a bearing system areused to move the wafer-supporting stage along a rail in the X-direction.To move the wafer-supporting stage in the Y-direction an intermediaterail is used that carries the rail and the wafer-supporting stage in theY-direction while allowing the wafer-supporting stage to slide along therail in the X-direction.

In addition to drive motors, various moving parts, bearings, and wafer,the wafer-supporting stage may carry a position detector. The positiondetector may be a laser-distance interferometer that is usually heavy,and requires environmental shielding of the optical path and slow motionin order to achieve high accuracy readings.

Hollis, U.S. Pat. No. 5,153,494, describes an apparatus capable ofmoving light loads in X, Y, and yaw (rotation around the Z axis) inlimited range and with high speed. Vibrations of the stationary portionsof the apparatus are reduced by a momentum-canceling design. Themomentum canceling design includes, in addition to the movablewafer-supporting stage, a non-coplanar moving element that moves in amomentum canceling manner with respect to the movable wafer-supportingstage. The motors include fixed permanent magnets and coils that areattached to the wafer-supporting stage and move along with thewafer-supporting stage.

Trumper, U.S. Pat. No. 5,196,745, describes an apparatus capable ofproviding movement in the 200 to 300 mm range in one or two degrees offreedom and having precision control in the 10 nm range in the remainingdegrees of freedom. Linear motion of the wafer-supporting stage isprovided by a motor having an array of permanent magnets attached to themovable wafer-supporting stage and a fixed array of commutating coilsattached to the stator. The commutating coils have coupled magnetic fluxlines. Current is allowed to flow only in those sections of the arraywhere the motion takes place, in order to reduce energy losses in thecoils. The motion is facilitated by magnetic bearings.

SUMMARY OF THE INVENTION

In one aspect, the invention features a device positioning apparatushaving a stage configured to support the device, a platen supporting thestage, and a plurality of drive motors, co-planar with the stage andarranged around the stage, that apply drive forces directly to the stagewithout any mechanical contact to the stage. The drive forces impartplanar motion to the stage in at least three degrees of freedom ofmotion. A plurality of fluid bearings operate between the stage and theplaten and constrain the planar motion of the stage in all remainingdegrees of freedom. A controller connected to the drive motors controlsoperation of the drive motors. Each drive motor has a top stationarycoil, a bottom stationary coil, and a movable magnet adapted to move ina gap of predetermined length formed in-between the top and bottomstationary coils.

The arrangement according to the invention makes it possible, in certainembodiments, for there to be only one monolithic movable part. Thisarrangement allows the stage to be made very stiff, which in turnpromotes use of very high servo gains in the controller. Benefits ofthis arrangement can include good precision, fast settling time, andsmall servo-tracking error.

Because the motors are provided in a movable magnet configuration, thedrive coils can remain stationary, thus eliminating any need to attachleads to the movable stage. This arrangement improves the reliability ofthe coils, because their leads are not flexed.

In some embodiments the stage may be a small-motion stage, and theapparatus may include a large-motion stage that moves on a planeparallel to a plane of motion of the small-motion stage. Thisarrangement allows a heavy load processing equipment, e.g., laser, to becarried by a low performance large-motion element that can scan the fullrange of the wafer. The light load wafer can be supported by ahigh-precision, high-speed, small-motion stage that scans the smallrange of the individual devices on the wafer.

In another aspect, the invention includes a force cancellation systemthat applies cancellation forces to the device positioning apparatus.The cancellation forces are co-planar with the center of gravity of thestage and any components that move with the stage and cancel forcesgenerated by the planar motion of the stage. Because both the driveforces and cancellation forces are on the same plane, no net moments arecreated out of the plane that may cause deflections in the stage and mayresult in lower precision. This planar configuration results in highservo loop stiffness, a fast settling time, and high precision.

In another aspect, the invention features a position detector thatincludes at least one interferometric encoder. The encoders areinsensitive to environmental disturbances such as change in temperature,humidity, or barometric pressure, thus reducing the need for controllingthe ambient environment of the positioning apparatus and therebyreducing cost. Interferometric encoders can be relatively inexpensive,have low weight, and can be configured in a moving grid configuration,which keeps the read head electronics and associated leads stationary.Reliability can be improved by eliminating lead flexing with movement ofthe stage.

In another aspect, the invention features a controller that controls theoperation of the drive motor and includes a trajectory planner thatreceives a description of a desired trajectory, produces stage drivecommands and force cancellation commands for the drive motors and theforce cancellation system, respectively, and compensates for phase lagbetween the stage drive commands and the force cancellation commands.The controller also includes a digital controller that receives commandsfrom the trajectory planner and feedback signals, and compares thefeedback signals to the commands and generates correction signals forthe drive motors.

Other features and advantages of the invention will be apparent from thefollowing detailed description, the drawings, and the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side view of a wafer positioning apparatus according to anembodiment of the invention.

FIG. 2 is a perspective view of the wafer positioning apparatus and thedrive and force cancellation motors of the apparatus of FIG. 1.

FIG. 3 is a perspective view of the stage of the wafer positioningapparatus.

FIG. 4 is a cross-sectional side view of part of the stages.

FIGS. 5 and 6 are diagrammatic views of the stage moving in theX-direction.

FIG. 7 is a side view of a portion of the stage and a portion of a drivemotor.

FIG. 8 is a view of the bottom surface of the stage.

FIG. 9 is a drawing of an interferometric encoder used in connectionwith the wafer positioning apparatus of FIG. 1.

FIG. 10 is a drawing of portions of the interferometric encoder, showingan optical path signal.

FIG. 11 is a schematic representation of three detector arrays of theinterferometric encoder, as well as detector array signals, and aninterference pattern associated with the detector arrays.

FIG. 12 is a block diagram of a stage control system that can be used tocontrol the stage of the apparatus of FIG. 1.

FIG. 13 is a block diagram of a digital controller of the stage controlsystem of FIG. 12.

DETAILED DESCRIPTION

Referring to FIG. 1, a semiconductor wafer positioning apparatus 100according to the invention includes a small-motion X-Y stage 102 thatcan be used to support, for example, a semiconductor wafer 106. Stage102 is capable of moving the wafer 106 over a small field in the X and Ydirections. In one example the field size is 35×35 millimeters squareand the precision of the motion is 10 nanometers.

Stage 102 is supported by a platen 104. Air bearings 144 a to 144 doperate between the stage and the platen. The stage/platen assembly isplaced on an optical table 124, which is mounted on vibration isolatedlegs 122.

Two large-motion stages 108 and 110 move an equipment box 116 along theX-axis and along the Y-axis, respectively. In one example, the largemotion field size is 300×300 millimeters.

The equipment box 116 may contain a microlithography laser 118, aninspection lens system 120, or other processing equipment. Thecombination of the stacked stages 102, 108, and 110, providesde-coupling of the large motion for the heavy load processing equipment118, 120 from the high-speed and high-precision small motion for thelight stage 102 that supports the wafer.

Stage 102 is movable in the X and Y directions, driven by two pairs ofdrive motors 112 a, 112 c, and 112 b, 112 d, respectively (FIG. 2). Thefour drive motors lie on the same plane as the stage and are arrangedsymmetrically around the stage on opposite sides of the stage. Theyprovide drive forces directly to stage 102 without any mechanicalcontact. In addition to the X and Y motion the motors can rotate thestage around the Z direction (yaw).

Referring to FIG. 3, stage 102 has four wings 130 a to 130 d. At thefree end of each wing, e.g., 130 a, there are two oppositely magnetizedpermanent magnets, e.g., 132 a and 134 a. The permanent magnets areplaced in a gap 160 formed between two stationary coils 140 and 142(FIG. 4). The stationary coils are formed on the surfaces of amagnetically permeable material 138, which acts as a return plate formagnetic flux lines 150 originating from the permanent magnets (FIG. 7).The permanent magnets form the armature of the motor and the stationarycoils with the return plate form the stator part of the motor. Because acurrent is applied to the stationary coils, a force is induced that actson the permanent magnets, causing them to move together with the stage.In one example, the permanent magnets may be Iron Neodymium Boron andthe magnetically permeable material may be soft iron.

Referring to FIGS. 5 and 6, the dimensions of the coils of the fourmotors are chosen to be wide enough so that any movement in onedirection (e.g., X) will maintain the flux lines of the permanentmagnets in the other direction (e.g., Y) fully coupled with the coils.

In the moving magnet configuration, the point at which the drive forceis applied to each wing of the stage remains stationary relative to thecenter of the movable stage. This arrangement results in a simple servocontroller.

With reference to FIG. 2, four drive motors are used to drive stage 102.In other embodiments three or more than four motors may be used (threemotors constitute the minimum need to control three degrees of freedom).The four-motor configuration can also utilize the minimum coil sizenecessary to cover a rectilinear field size. A short coil length resultsin low power losses.

With reference to FIGS. 4 and 7, the drive coil is split into two coils:top coil 140 and bottom coil 142. The two coils 140, 142 may beconnected in series or in parallel, or may be driven independently ofeach other in order to provide control over the forces generated by thetwo halves of the drive coil. This arrangement can be used to createsmall roll (rotation around the X axis) and pitch (rotation around the Yaxis) motions in addition to the X, Y and yaw. Active control of rolland pitch motions can be used to compensate for imperfections inmanufacturing tolerances that could lead to misalignment of the stagewhile ensuring that no net moments are applied to the air bearings 144a-144 d (FIG. 8) as a result of motor forces. If coils 140, 142 aredriven independently of each other, the ratio of current in top coil 140relative to the current in bottom coil 142 can be controlled so as toensure that the resultant net force is applied in the plane of thecenter of gravity of the moving components.

Heat is generated at the coils 140, 142. Because the coils are bonded tothe magnetically permeable material 138, they can be cooled withoutdisturbing motion of the stage and any precision metrology that may beembedded in the stage.

The stators, including the top and bottom coils 140, 142 and themagnetically permeable material 138, are kinematically mounted to theplaten. This construction allows the stators to change size due tothermal expansion without stressing the platen, and reduces heatconduction from the stators to the platen, thereby allowing thetemperature control of the stators to be simple and inexpensive. In analternative construction, the platen is kinematically mounted to asub-frame that is allowed to expand thermally without affectingprecision metrology.

Referring to FIG. 8, air bearings 144 a to 144 d operate between thebottom surface 170 of stage 102 and the top surface of platen 102. Thestiffness of the air bearings is improved by pre-loading the bearingswith a vacuum 146 drawn in the gap 180 between stage 102 and the platen104, as shown in FIG. 1. Because the air bearing stiffness iscontrolled, the small-motion stage is constrained in the Z direction andwith respect to rotation around the X-axis (roll) and around the Y-axis(pitch).

Air bearings provide a frictionless support between the stationaryplaten and the stage that is free of high-spatial frequency kinematicerrors. A one-time metrology process can easily map the small amplitudeof built-in form errors that do exist in the platen.

The movable magnet configuration facilitates a large fluid bearingpre-load by offsetting the magnets in the vertical gap between the drivecoils and return flux paths. The extra pre-load provided by the magnetscontributes to improved bearing stiffness, which has the benefits ofgood precision, fast settling time, and small servo-tracking errors.

The plane of action of the drive motors is matched to the planecontaining the center of gravity of the moving components. Forces thatdrive the center of gravity of the moving components do not causeundesirable deflections of the fluid bearings. The de-coupling betweendrive forces and fluid bearing deflections improves servo loop stiffnessand reduces the time required to achieve high precision following anacceleration.

The air bearings are not over-constrained. An over-constrainedair-bearing is difficult to manufacture due to precise tolerances thatmust be maintained over relatively large distances. Because the bearingsdo not require precision tolerances in their manufacture, their cost canbe low.

One method of position feedback is based on the use of interferometricencoders 136 a to 136 d placed at the bottom surface 170 of the stage,as shown in FIG. 8. Referring to FIG. 9, an interferometric encoderincludes a two-dimensional encoder grid 182, a sensor head 184, and aconnector 186 connecting the sensor to an interface card 188, whichplugs into a computer (not shown). The two-dimensional grid 182 has a10-micrometer scale etched on borosilicate glass. The grid is attachedto the movable stage. The sensor head 184 is attached to the platen andincludes, as shown in FIG. 10, a laser diode source 192, lenses 194,prism-mirrors 196, and detectors 198 a, 198 b. The optical path betweenthe grid and the sensor head is enclosed. The laser 192 directs a lightbeam 200 focused through the lenses 194 to the grid 182. The beam 200 isdiffracted by the grid and interference patterns 202 a and 202 b areformed from the two separate dimensions of the grid. Each of the twointerference patterns 202 a and 202 b is imaged onto detector arrays 198a and 198 b. The detector arrays generate signals that can be processedto make accurate phase measurements. When the diffraction grating movesrelative to the sensor head the interference patterns move across thedetector arrays, generating, as shown in FIG. 11, signals R, S, and T,which are 120 degrees apart in phase. These signals are electronicallyprocessed by a computer to provide accurate measurement of the phase.This technique results in phase resolution of 1 part in 2¹⁴, whichcorresponds to a length resolution of 0.3 nm.

The interferometric encoders may be manufactured by, for example, MicroE (Massachusetts) or Optra, Inc. (Topsfield, Mass.).

The encoders are insensitive to environmental disturbances such aschange in temperature, humidity, or barometric pressure, thuseliminating any need for controlling the ambient environment of thewafer positioning apparatus and thereby providing low cost. The enclosedoptical path for the encoders beneath the stage is free of turbulence,thus allowing the positioning electronics to operate at a highbandwidth, which allows higher servo loop stiffness.

Interferometric encoders are relatively inexpensive, have low weight,and are configured in a moving grid configuration, which keeps thesensor head electronics and associated leads stationary. Because thereis no lead flexing with movement of the stage, reliability is high.

The precision of the encoders is locked into the encoder grid. There maybe errors in the grid, but these errors can be measured once and thecorrection can be part of the metrology control system. A look-up tablethat is established once when the wafer positioning apparatus isassembled can easily correct the grid errors.

Position feedback may also be implemented by conventional laserinterferometer means.

In order to cancel forces caused by the movements of the stage, a forcecancellation system is provided as an integral component of the waferpositioning apparatus. Referring again to FIG. 2, the force cancellationsystem includes four force cancellation motors 114 a to 114 d co-planarwith stage 102 and arranged around the stage. The cancellation motorsapply cancellation forces to stationary portions of the waferpositioning apparatus, which are directed opposite to the forcesgenerated by the planar motion of the stage.

Integral force cancellation allows much higher accelerations and lesswait time after acceleration intervals and before full system accuracyis achieved. The force canceling capability also helps to prevent thestage from shaking any precision components that move with the stage.

The plane of action of force cancellation is also located in the sameplane that will contain the center of gravity of the moving parts of theapparatus (stage 102 combined with any component that the stage isdesigned to support and move). Because both the drive forces andcancellation forces are on the same plane, no net moments are createdout of the plane that may cause deflections in the stage and may resultin lower precision. This planar configuration results in high servo loopstiffness, fast settling, and high precision.

Referring to FIG. 12 the main components of control system 50 are thetrajectory planner 200 and the digital controller 210.

The trajectory planner 200 receives a description of a desiredtrajectory 202 and produces set points for the stage position servo, andthe corresponding force feed-forward commands for both the stage drivemotors 230 and the force cancellation motors 240. The trajectory planneris configured to compensate for phase lag between the stage drivefeed-forward commands and the force cancellation feed-forward commands.

The digital controller 210 receives the stage position and feed-forwardcommands 204 for the drive and force cancellation motors from trajectoryplanner 200. The digital controller 210 also receives feedback signals244, 234, and 211 from force cancellation sensors 242, stage positionsensors 232, and drive motor analog controllers 212, respectively. Thedigital controller processes the sensor signals 242, 234, and 211 todetermine the state of the wafer positioning apparatus, compares thestate to the desired state, and generates the proper correction signals213 and 215, which are applied to analog controllers 212 and 214. Thedigital controller also provides the state information 246 to aparameter estimator 220.

Analog controllers 212 and 214 control the currents to stage drive motorcoils 216 and the force cancellation motor coils 218, respectively.Drive motor analog controller 212 provides a drive motor feedback signal211 to the digital controller. Drive motor feedback signal 211 is anestimate of the velocity of the stage calculated using a model of thedrive motor and measurement of the drive coil voltage and current.

Analog controllers 212 and 214 provide relatively large currents 224 and222 required to drive the linear drive and force cancellation motors,respectively. Each analog controller has linear dual output stages. Thedual output stages can be connected in parallel to provide double thedrive current into a low impedance coil. The dual outputs can also bedriven in a master/slave configuration in order to drive the top/bottomcoils of each stage drive motor. In one example, the slave output maydeliver a multiple between 0.95 and 1.05 of the current delivered by themaster. This fixed ratio can be adjusted to compensate for manufacturingtolerances in the wafer positioning apparatus and the drive motors andthereby ensure that the net drive force applied by each drive motor isapplied at the center of gravity of the moving components.

The digital controller receives stage position feedback 234 from stagedrive system 230 through four channels of position sensors 232 (encodersor interferometers). Because the stage has only three degrees offreedom, one channel of feedback is redundant. This redundancy isexploited in the digital controller to derive a best-fit estimate ofstage position and also to provide an estimate of the quality of thesensor signals. The quality of the sensor signals is simply the residualbetween the best-fit estimate of stage position and the fourmeasurements from which the fit was estimated. The digital controllersoftware monitors the quality measurement and will execute shutdownprocedures if the quality exceeds preset limits.

Vibration sensors 242 (either geophones or accelerometers) are includedon the wafer positioning apparatus to monitor the force cancellationperformance. A calibration system 260 monitors the force cancellationperformance during the manufacture of the wafer positioning apparatusand adjusts the feed-forward gains 208 (static parameters).

A parameter estimator 220 is also provided in the controlling software.The parameter estimator 220 looks at the desired trajectory and theactual state variables of the stage drive system and force cancellationsystem and derives an improved estimate 206 of the parameters to be usedby the trajectory planner 200. The goal of the parameter estimator is todrive the dynamic error of the stage to be near zero at all times. Forthis condition to occur, the trajectory planner 200 must provide forcecommands that correspond nearly perfectly to the desired positiontrajectory. Nearly perfect correspondence requires excellent knowledgeof amplifier gains, stage mass, motor location relative to the stage,and similar parameters. Errors in the model of the wafer positioningapparatus may lead to tracking errors when the stage executes a motionprofile. As the system model gets better, due to operation of theparameter estimator 220 leading to continuous improvement in accuracy ofthe model, the stage will execute a trajectory with less settling time.In addition to improved performance, the limits on tracking errors canbe set ever tighter in the digital controller as the model is improved.Very tight tolerances on tracking errors allow the digital controller todetect errors, collisions, or other unanticipated events faster, whichensures good safety of the system.

The calibration system 260 measures and corrects geometric errors instage position sensors 232 and establishes baseline parameter valuesthat will be subsequently improved by the parameter estimator. Thecalibration system 260 can also be used to make off-line calibration.For off-line calibration the calibration system 260 may use additionalsensors and software.

The trajectory planner 200 is designed to perform specific tasks. Forthe task of re-positioning a semiconductor element for the purpose ofmemory repair, for example, the trajectory is specified as a sequence ofsegments. The segments include a Position/Velocity/Time segment, and aCruise-velocity/Distance segment.

The Position/Velocity/Time segment specifies initial and final positionand velocities and the time allotted to perform the movement. Thetrajectory planner determines the position and corresponding forceprofiles necessary to satisfy the end-point conditions. The path takenalong the movement is not constrained. A special condition of zero timeindicates that the trajectory planner should use the minimum timenecessary for the movement subject to actuator or other limitations.

The Cruise-velocity/Distance segment is used to perform most processingoperations. Only the length of this segment is specified. The velocityfor the segment is specified by the endpoint of the preceding segment.The Cruise-velocity/Distance segment cannot be the first segment in atrajectory. Processing commands may be linked to theCruise-velocity/Distance segment type.

Other types of segments may be used in other applications.

The trajectory planner 200 expands a trajectory specified as an orderedlist of trajectory segments into a list of trajectory points that willbe issued at a pre-determined update rate to the digital controller. Thetrajectory planner operates in near real-time at a slight previewinterval to the actual motion. A trajectory buffer 300 (FIG. 13) existsbetween the trajectory planner, the digital controller and the parameterestimator. The trajectory buffer 300 allows the rate of execution of thetrajectory planner to be de-coupled from the digital controller rate.The trajectory planner must run sufficiently often to ensure that thetrajectory buffer is never empty. There is a very slight delay at thestart of a motion sequence that allows the trajectory planner to get farenough ahead of the digital controller that the buffer will not beempty. After the initial start-up delay (a few milliseconds), the timerequired to perform a motion is completely dependent upon the physicallimitations of the hardware. There are no further software induceddelays in the system.

The trajectory planner uses closed-form motion profiles in order toconstruct the trajectory segments. In one example, a linearly varyingcycloidal acceleration profile is used for small motion in order toimplement a Position/Velocity/Time segment type:

A(t)=(sin(π*t/T)*(k ₀ +k ₁ *t)

Where:

A is Acceleration profile (or equivalently, force profile)

k₀ and k₁ are profile constants that are used to match trajectoryendpoints

T is segment duration (time)

t is time (independent variable)

This profile is a smoothly changing force profile over the course of thesegment and has sufficient flexibility to meet the end-point constraintsof the trajectory segment. Because the profile is smooth, thefeed-forward force signals that correspond to the profile are smoothtoo. This result is beneficial because the feed-forward force signalsare applied directly to the amplifiers and motors without additionalfiltering. Because the feed-forward force signals are applied in anopen-loop configuration, the dynamics of the wafer positioning apparatusdo not influence the applied force signals, thereby providingdramatically simple system dynamics.

Referring to FIG. 13 the key elements of the digital controller 210 area trajectory buffer 300, a command buffer interface 302, a monitorbuffer interface 304, a feedback transform 306, an error detection block308, a control law 310, an output transform 314, and a motor statefeedback block 312.

The trajectory planner passes trajectory commands to the digitalcontroller via trajectory buffer 300. Also, the digital recorder passesback data to parameter estimator 220 via trajectory buffer 300. Thisarrangement is implemented using a block of shared memory that can beaccessed by both the low level processor (digital controller) as well asthe higher level processor (trajectory planner and parameter estimator).

The command buffer interface 302 extracts force feed-forward andposition set-point commands from trajectory buffer 300 and uses them ineach update of the digital controller.

Monitor buffer 304 enters state information to parameter estimator 220by inserting the measured data into trajectory buffer 300.

Four channels of stage position are measured through stage sensorinterface 316. The four sensor readings are converted into a best fitestimate of the three degrees of freedom stage position (X, Y, yaw) byfeedback transform 306. The quality of the sensors is also computed andmonitored, along with additional measurements from the amplifier, byerror detection block 308. Error detection block 308 initiates shutdownprocedures whenever a failure is detected based on out-of-limitsmeasurements. Shutdown may be initiated for poor sensor quality (e.g.,excessive residuals in the feedback transform), if the position errorexceeds a preset limit, if the estimated stage velocity exceeds a presetlimit, and if the motor temperature or current exceeds a preset limit.

Control law 310 receives as input a three-degree-of-freedom positionerror signal and additional state information from the motors andproduces three force commands that are designed to drive the trackingerror to zero. The three forces resulting from control law 310 aretransformed by output transform 314 into a minimum power vector that isapplied in the moving magnet coordinate system by the four stage drivemotors. The error forces are summed, in the moving magnet coordinatesystem, with the feed-forward force commands coming directly from thetrajectory buffer. The resulting vector sum of forces is sent to thefour analog controllers.

Stage motor state feedback block 312 monitors feedback from the analogcontroller. Monitored variables include motor current, armaturevelocity, and stator temperature.

The stage control system provides highly predictable response of thestage to force commands. The high degree of predictability allows theestimation of feed-forward forces to be made with great accuracy.

The actual force required to drive the stage may be slightly differentfrom the predicted force profile. However, because of the high servoloop gains (which are also possible due to the simplicity of the waferpositioning apparatus and the absence of low frequency vibration modes),any errors in the predicted forces can be corrected with minimaltracking error. In one example, the required force profile may bepredicted with better than 99 percent accuracy directly from thefeed-forward path. Tracking error multiplied by the servo loop gain willgenerate the residual force required to drive the stage along aprescribed trajectory.

Because the vast majority of the forces (both drive and forcecancellation) are predetermined, very effective force canceling ispossible. This high degree of cancellation is possible withoutinteraction or coupling of the system dynamics.

The use of the feed-forward model to generate the majority of the drivesignal provides good safety. Because the closed loop servo controllersupplies very little force, it is possible to set a very tight toleranceon position error. The tight tolerance on servo error allows faults tobe detected and shutdown procedures to commence before substantialdamage can be done to the system or any personnel. It is easy to detectwhen a maintenance person, for instance, applies an external disturbanceto the wafer positioning apparatus such as via unanticipated touching.

Another advantage of the control architecture is the partitioning of thecomputation load amongst multiple processors, such as the trajectoryplanner and the digital controller. The digital controller is optimizedto provide low-latency (measured in microseconds) computation in ahighly deterministic faction in order to satisfy closed loop stabilitycriteria. The trajectory planner operates with low deterministicbehavior (latency measured in tens to hundreds of milliseconds). While asingle processor with a suitable operating system could perform bothfunctions, the partition facilities using multiple processors with ashared memory interface between them. This partition allows a dedicateddigital signal processor to be used for the digital controller and ahost with near real-time in performance (such as UNIX or Windows NT) toimplement the trajectory planer.

There has been described novel and improved apparatus and techniques forhigh-speed precision positioning. The invention is to be construed asembracing each and every novel feature and novel combination of featurespresent in or possessed by the apparatus and technique herein disclosed.It is also evident that those skilled in the art may now make numeroususes and modifications of and departures from the specific embodimentsdescribed herein without departing from the inventive concept.

What is claimed is:
 1. A device positioning apparatus comprising a stageconfigured to support the device, a platen supporting the stage, atleast three drive motors co-planar with the stage and arranged aroundthe stage that each apply a drive force directly to the stage withoutany mechanical contact to the stage, the drive forces imparting planarmotion to the stage in at least three degrees of freedom of motion, aplurality of fluid bearings operating between the stage and the platen,the fluid bearings constraining the planar stage motion at least oneremaining degree of freedom, a controller connected to the drive motorsand configured to control operation of the drive motor, and wherein eachdrive motor comprises a top stationary coil, a bottom stationary coil,and a movable magnet adapted to move in a gap of predetermined lengthformed in-between the top and bottom stationary coils, each drive motorhaving at most one top stationary coil and at most one bottom stationarycoil, and each drive motor having at least one distinct movable magnet.2. The apparatus of claim 1 the three drive motors are symmetricallyarranged around the stage.
 3. The apparatus of claim 1 comprising fourdrive motors symmetrically arranged on four opposite sides of the stage,that apply drive forces imparting motions to the stage in threeindependent degrees of freedom.
 4. The apparatus of claim 1 comprisingat least three fluid bearings.
 5. The apparatus of claim 1 wherein thefluid bearing comprises a gas pressurized bearing.
 6. The apparatus ofclaim 1 wherein heat is removed from the top and bottom coils byconduction to a fluid stream.
 7. The apparatus of claim 1 wherein thetop and bottom coils are independently driven.
 8. The apparatus of claim1 wherein the movable magnet comprises a permanent magnet.
 9. Theapparatus of claim 8 wherein the permanent magnet comprises an IronNeodymium Boron magnet.
 10. The apparatus of claim 1 wherein the movablemagnet of each drive motor comprises two oppositely magnetized magnets.11. The apparatus of claim 1 wherein the movable magnet comprises anelectromagnet.
 12. The apparatus of claim 1 wherein the stage is asmall-motion stage, further comprising a larger-motion stage configuredto move on a plane parallel to a plane of motion of the small-motionmovable stage.
 13. The apparatus of claim 1 wherein the top and bottomstationary coils of each motor have larger dimensions than the movablemagnet so that flux lines from the movable magnet are always fullycoupled to the coils throughout the entire range of motion of the stage.14. A device positioning apparatus comprising a stage configured tosupport the device, a platen supporting the stage, a plurality of drivemotors co-planar with the stage and arranged around the stage that applydrive forces directly to the stage without any mechanical contact to thestage, the drive forces imparting planar motion to the stage in at leastthree degrees of freedom of motion, a plurality of fluid beatingsoperating between the stage and the platen, the fluid bearingsconstraining the planar stage motion at least one remaining degree offreedom, a controller connected to the drive motors and configured tocontrol operation of the drive motors, and wherein each drive motorcomprises a top stationary coil, a bottom stationary coil, and a movablemagnet adapted to move in a gap of predetermined length formedin-between the top and bottom stationary coils, and a sub-frame, whereinthe sub-frame supports the top and bottom coils, and the platen iskinematically connected to the sub-frame so that the coils can changesize due to thermal expansion without applying stresses to the platen.15. A device positioning apparatus comprising a stage configured tosupport the device, a plurality of at least three drive motors co-planarwith the stage and arranged around the stage that each apply a driveforces force directly to the stage without any mechanical contact to thestage, the drive forces imparting planar motion to the stage in at leastthree degrees of freedom of motion, wherein each drive motor comprises atop stationary coil, a bottom stationary coil, and a movable magnetadapted to move in a gap of predetermined length formed in-between thetop and bottom stationary coils, each drive motor having at most one topstationary coil and at most one bottom stationary coil, and each drivemotor having at least one distinct movable magnet, a force cancellationsystem configured to apply cancellation forces, co-planar with a centerof gravity of the stage and any components that move with the stage,that cancel forces on the stage that are generated by the planar stagemotion, and a controller connected to the drive motors and configured tocontrol operation of the drive motors and the force cancellation system.16. The apparatus of claim 15 wherein the force cancellation systemcomprises a plurality of force cancellation motors co-planar with thestage and drive motors and arranged around the stage, that applycancellation forces to stationary portions of the device positioningapparatus, directed opposite to the forces generated by the planar stagemotion.
 17. A device positioning apparatus comprising a stage configuredto support the device, at least three drive motors, each providing aforce directly to the stage, the forces imparting motion to the stage inat least three degrees of freedom of motion, each of the drive motorscomprising a top stationary coil, a bottom stationary coil, and amovable magnet adapted to move in a gap of predetermined length formedin-between the top and bottom stationary coils, each drive motor havingat most one top stationary coil and at most one bottom stationary coil,and each drive motor having at least one distinct movable magnet; acontroller configured to control operation of the drive motors, and aposition detector providing a feedback signal to the controllerrepresenting position of the stage, the detector comprising at least oneinterferometric encoder.
 18. A method of operating a device positioningapparatus, comprising the steps of: imparting motion to a large-motionstage that supports device processing equipment, comprising laserequipment, so as to cause the device processing equipment, comprisingthe laser equipment, to move with the large-motion stage; repositioninga device to be processed by the device processing equipment byrepositioning a small-motion stage in at least one degree of freedom, bycontrolling operation of at least one drive motor that provides force tothe small-motion stage to reposition the stage, the small-motion stagebeing repositioned from an initial position to a final position in aplane parallel to a plane of motion of the large-motion stage, thesmall-motion stage supporting the device to be processed by the deviceprocessing equipment; and processing the device using the deviceprocessing equipment; the motion imparted to the large-motion stagebeing a large motion that is de-coupled from a high-speed andhigh-precision small motion of the small-motion stage.
 19. The method ofclaim 18 wherein the device processing equipment comprises a laser andthe device is a semiconductor device.
 20. A device positioning apparatuscomprising: device processing equipment comprising laser equipment; alarge-motion stage configured to the support device processing equipmentcomprising laser equipment, and to impart motion to the deviceprocessing equipment in a plane of motion so as to cause the deviceprocessing equipment, comprising the laser equipment, to move with thelarge-motion stage; a small-motion stage configured to support thedevice and to reposition the device from an initial position to a finalposition in a plane parallel to a plane of motion of the large-motionstage; at least one drive motor that provides force to the small-motionstage to reposition the stage in at least one degree of freedom; and acontroller configured to control operation of the at least one drivemotor so as to reposition the small-motion stage; the device processingequipment being configured to process the device; the motion imparted tothe large-motion stage being a large motion that is de-coupled from ahigh-speed and high-precision small motion of the small-motion stage.21. The apparatus of claim 20 wherein the laser equipment is a laser.22. The apparatus of claim 20 the laser equipment is laser processingequipment other than a laser itself.
 23. A device positioning apparatuscomprising: device-processing equipment; a stage configured to supportthe device for processing by the device-processing equipment; at leastone stage driver providing force to the stage, the force impartingmotion to the stage, relative to the device-processing equipment, in atleast two degrees of freedom; a position detector providing a signalrepresenting position of the stage; a trajectory planner that receives adescription of a desired trajectory between positions at which thedevice is to be processed by the device-processing equipment, expandsthe desired trajectory into a list of trajectory points between thepositions at which the device is to be processed by thedevice-processing equipment, at least some of the trajectory pointsbeing distinct from positions at which the device is to be processed,and produces stage drive commands for the stage driver based on thedesired trajectory; and a comparator that receives the list oftrajectory points from the trajectory planner, receives signals from theposition detector, and compares the signals from the position detectorto the trajectory points received from the trajectory planner todetermine the state of the device positioning apparatus.
 24. Theapparatus of claim 23 wherein the comparator is a digital controllerthat generates correction signals for the stage driver.
 25. Theapparatus of claim 23 wherein the stage driver is a drive motor.
 26. Theapparatus of claim 25 wherein the comparator is a digital controllerthat generates correction signals for the driver motor, and furthercomprising a drive motor controller that provides signals to the digitalcontroller representing operation of the drive motor.
 27. The apparatusof claim 23 further comprising a force cancellation system that performsforce cancellation with respect to the stage and in response to forcecancellation commands.
 28. The apparatus of claim 27 wherein thetrajectory planner produces force cancellation commands for the forcecancellation system.
 29. The apparatus of claim 27 wherein thecomparator is a digital controller that generates correction signals forthe force cancellation system.
 30. The apparatus of claim 29 furthercomprising a parameter estimator that receives system state feedbacksignals from the digital controller comprising stage position and forcecancellation state feedback, compares them to the desired trajectory,and derives improved estimates of parameters used by the trajectoryplanner to generate commands received by the digital controller.
 31. Theapparatus of claim 23 wherein the position detector comprises at leastone interferometric encoder.
 32. The apparatus of claim 23 wherein thedevice-processing equipment comprises a laser.
 33. The apparatus ofclaim 23 further comprising an inspection lens system.
 34. The apparatusof claim 23 further comprising: a large-motion stage configured to thesupport device processing equipment and to impart motion to the deviceprocessing equipment in a plane of motion; wherein the stage thatsupports the device for processing by the device-processing equipment isconfigured to impart motion to the device in a trajectory in a planeparallel to a plane of motion of the large-motion stage.
 35. Theapparatus of claim 34 wherein the large-motion stage is furtherconfigured to support an inspection lens system.
 36. The apparatus ofclaim 35 wherein the inspection lens system comprises a high-resolutionelectron microscopy system.
 37. The apparatus of claim 32 wherein thedevice for processing by the device-processing equipment is a siliconwafer and wherein the stage is configured to support the silicon waferfor processing by the laser.
 38. The apparatus of claim 33 wherein theinspection lens system comprises a high-resolution electron microscopysystem.
 39. A device positioning apparatus comprising: device-processingequipment; a stage configured to support the device for processing bythe device-processing equipment; at least one stare driver providingforce to the stage, the force imparting motion to the stage, relative tothe device-processing equipment, in at least two degrees of freedom; aposition detector providing a signal representing position of the stage;a trajectory planner that receives a description of a desired trajectorybetween positions at which the device is to be processed by thedevice-processing equipment, expands the desired trajectory into a listof trajectory points between the positions at which the device is to beprocessed by the device-processing equipment, and produces stage drivecommands for the stage driver based on the desired trajectory; acomparator that receives the list of trajectory points from thetrajectory planner, receives signals from the position detector, andcompares the signals from the position detector to the trajectory pointsreceived from the trajectory planner to determine the state of thedevice positioning apparatus; and a force cancellation system thatperforms force cancellation with respect to the stage and in response toforce cancellation commands; wherein the trajectory planner producesforce cancellation commands for the force cancellation system andwherein the trajectory planner is configured to compensate for phase lagbetween the stage drive commands and the force cancellation commands.40. A device positioning apparatus comprising: device-processingequipment; a stage configured to support the device for processing bythe device-processing equipment; at least one stage driver providingforce to the stage, the force imparting motion to the stage, relative tothe device-processing equipment, in at least two degrees of freedom; aposition detector providing a signal representing position of the stage;a trajectory planner that receives a description of a desired trajectorybetween positions at which the device is to be processed by thedevice-processing equipment, expands the desired trajectory into a listof trajectory points between the positions at which the device is to beprocessed by the device-processing equipment, and produces stage drivecommands for the stage driver based on the desired trajectory; acomparator that receives the list of trajectory points from thetrajectory planner, receives signals from the position detector, andcompares the signals from the position detector to the trajectory pointsreceived from the trajectory planner to determine the state of thedevice positioning apparatus; a force cancellation system that performsforce cancellation with respect to the stage and in response to forcecancellation commands, wherein the comparator is a digital controllerthat generates correction signals for the force cancellation system; anda sensor that provides signals to the digital controller representingeffectiveness of force cancellation.
 41. The apparatus of claim 40wherein the sensor is a vibration sensor.
 42. A method of operating adevice positioning apparatus comprising: supporting on a stage a devicefor processing by device-processing equipment; providing force to thestage through at least one stage driver, the force imparting motion tothe stage, relative to the device-processing equipment, in at least twodegrees of freedom; providing a signal from a position detector, thesignal representing position of the stage; controlling operation of thestage driver by a trajectory planner that receives a description of adesired trajectory between positions at which the device is to beprocessed by the device-processing equipment; expands the desiredtrajectory into a list of trajectory points between the positions atwhich the device is to be processed by the device-processing equipment,at least some of the trajectory points being distinct from positions atwhich the device is to be processed; and produces stage drive commandsfor the stage driver based on the desired trajectory; receiving, at acomparator, the list of trajectory points from the trajectory planner;receiving, at the comparator, signals from the position detector;comparing, at the comparator, the signals from the position detector tothe trajectory points received from the trajectory planner to determinethe state of the device positioning apparatus; and processing the deviceusing the device-processing equipment.